Soft Lithography and Unconventional Nanofabrication

Unconventional Nanofabrication

Nanofabrication is the process of making nanostructures that have arbitrary patterns with interesting physical properties. While we have pioneered many unconventional nanofabrication techniques in the past (Figure 1) – including soft-lithographic methods based on elastomeric stamps (micro-contact printing) or stencils (dry resist and lift-off with PDMS membranes), original photolithoghraphic methods based on arrays microlenses obtained by reflowing polymeric posts or by using polystyrene microspheres (microlens lithography), and methods based on thin sectioning of vertical structures to prepare thin layers of materials with controlled lateral dimensions as small as one nanometer (nanoskiving) – we have focused, in the last few years, on the development of unconventional nanofabrication approaches for the design, preparation, and screening of optical metamaterials.

The limitations of conventional approaches have limited the efficiency of exploratory research in this field, to the point that fabrication has been the limiting step for innovation. For instance, serial techniques such as focused ion beam (FIB), or electron beam lithography (EBL) provide ultra-high resolution, but are inefficient when one needs to prepare many samples in a short amount of time. These techniques, therefore, only permit one to fabricate specific targeted sample geometries for which the properties are typically predicted beforehand. A further, and very important, limitation that is worth noting is that access to such fabrication equipment can be extremely limited, or non-existent at some research institutions.

Based on those premises, we seek to develop low-cost, fast, parallel, high-resolution unconventional fabrication techniques that allow for greater flexibility in patterning at resolutions relevant to optical applications (in either the visible, or the NIR to MIR range).

Our current efforts are centered on two techniques:

- Shadow sphere lithography (SSL)

- Microlens lithography (µLens)

Both of those techniques employ microspheres as a mean to define submicron patterns by either casting shadows in a physical vapor deposition (PVD) process, or by casting focused UV light patterns on a photoresist (µLens).

Shadow Sphere lithography

We recently developed a method that we term Shadow Sphere Lithography (SSL), which brings together the concepts of nanosphere lithography – a method popularized by Van Duyne in which close-packed colloidal microspheres are used as a deposition mask for PVD – and our own interest for shadow deposition which had led us to prepare fully functional transistors with a single lithographic step. In SSL, the controlled angled PVD through a universal colloidal mask allows us to generate a nanoscale pattern with very high fidelity and very high resolution (see Figure 2).

SSL allows us to perform high-throughput fabrication of complex metamaterial structures with sufficient quality for optical applications, and allows us to fabricate multimaterial nanostructures in a single pump-down cycle of the electron beam evaporator, by simply changing the materials and adjusting the angles of evaporation without having to remove the sample from the chamber. The ease of use of the method and the natural polycristallinity of the self-assembled colloidal masks provide both a fast iteration between designs, and a natural opportunity for discovery.

Following the same principles, we are currently working on extending this technique to the fabrication of metamaterials with lattices that are different from a simple hexagonally close-packed arrangement.

Microlens Lithography

Some years ago, we developed microsphere lithography as a technique in which the image of a microscopic mask is reproduced by projection photolithography by an array of microlenses. In this process, the projection is done at a reduction factor of c.a. 10000x, converting a centimeter size feature in the masks to a micron size feature on the substrates.

Every microlens in the array projects an identical image of the mask, and the iteration from sample to sample can be done in a matter of minutes, with the limitation for the fabrication of a new structure being limited by the fabrication of a mask, which can be easily accomplished on a commercial laser cutter.

We are now developing and optimizing methods to produce non-periodic plasmonic arrays of nanostructures by microlens lithography.